Passive intermodulation known as “PIM” is a fairly well known type of electromagnetic interference in cellular systems. Downlink signals at the cell site mix at passive, non-linear junctions in the radio frequency (RF) path, creating new signals. If these new signals (intermodulation products) fall in an operator's uplink band, they can elevate the noise floor and degrade system performance. Mobile operators require testing at multiple steps in the process to detect and mitigate PIM. Individual components are tested by manufacturers before leaving the factory using factory PIM test equipment in accordance with standard IEC 62037 promulgated by the International Electrotechnical Commission (IEC) to verify that the individual components are low PIM.
In conventional PIM testing procedures, two test signals (F1, F2) corresponding to cellular telephone downlink bands are generated by a PIM analyzer, combined and transmitted to the device under test. PIM produced by the device under test propagates in all directions. PIM returning to the PIM analyzer passes through a series of filters and a low noise amplifier before arriving at the test instrument receiver.
Special care must be taken when PIM testing through antennas to make sure that the environment around the antenna is “PIM-free.” PIM generated by sources beyond the site antenna will radiate back into the site antenna and will be detected by the PIM analyzer receiver. Even antennas with very good PIM performance based on its own PIM production will fail the PIM test when measured in an environment with external PIM sources.
After a cellular installation is completed, a system level PIM test is conducted using field PIM test equipment. This test verifies that the entire RF path is low PIM, including the antenna feed system as well as the environment around the antenna. Failures due to PIM sources inside the antenna feed system are relatively easy to locate. Software features are available from PIM test equipment manufacturers to identify how far the offending PIM source(s) are located from the test analyzer. This information combined with the knowledge that PIM usually occurs at RF connections enables technicians to quickly locate internal PIM sources and make repairs.
If the PIM source is beyond the antenna itself, however, identifying the location of the PIM source becomes significantly more difficult. Distance-to-PIM estimates may provide an indication of how far the PIM source is from the antenna, but does not provide any guidance on the direction. The method most commonly used to locate external PIM problems has been a “trial and error” method using RF absorbing foam. The process involves measuring system PIM while methodically covering metal objects in front of or near the back of the antenna with RF absorbing foam until the technician either gets lucky and finds all the problems, or runs out of RF absorber. This method is very time consuming and ultimately may not be successful if the offending PIM source is not somewhere obvious.
There is, therefore, a need for a more effective approach for PIM detection and mitigation.